1. The Field of the Invention
The present invention relates generally to a method and apparatus for securely attaching a terminal to a conducting wire. More particularly, embodiments of the present invention relate to an improved applicator die that reliably establishes and maintains the terminal in a desired orientation during processing positively releases the wire/terminal assembly after crimping is complete.
2. The Prior State of the Art
A multitude of approaches have been taken to address the problem of connecting an electrical wire to a terminal component. Some approaches focus on a connecting means formed on or in the terminal component. Typically, these connecting means are adapted to receive the bare end of a conducting wire. That is, no special treatment to the end of the wire is required to form the desired electrical connection. One example of such a connecting means is an electrically conductive post about which a wire is wrapped. Typically, the post is threaded so that a nut can advance down the post so as to pinch the wire between the nut and a plate at the bottom of the post. Another example of a connecting means adapted to receive a bare wire is found in light switches and audio speakers. These devices typically employ an orifice partially closed off by a spring-loaded conductor. In operation, the bare wire is inserted into the orifice, pushing the spring-loaded conductors aside. Once the wire is fully inserted, the spring force exerted by the spring-loaded conductors acts to retain the wire in place.
While the `bare wire` type of electrical connectors is still preferred for certain applications, it is recognized that in other applications, it would be desirable to modify the end of the wire in some way so as to achieve a desired type of connection. Specifically, it is desired to attach to the end of the conducting wire a terminal that is adapted to readily mate with a corresponding connector on an electrical component. To that end, a wide variety of terminal types have been developed. A typical terminal comprises an opening in which a conducting wire is inserted, and a connecting portion integral with the structure of the opening. The opening is usually in the form of a closed loop (also known as a "barrel"), or an open loop.
Obviously, however, the terminals needed to be reliably secured to the conducting wire. Selection of a method or methods for securing terminals is driven by two primary considerations. First, the connection must be mechanically strong and reliable so that thermally induced expansion and contraction does not compromise the performance of the wire/terminal assembly. Furthermore, a mechanically strong connection ensures that the wire will not pull out of the terminal if either the wire or terminal is subjected to stress. Second, the method used to connect the terminal to the conducting wire must produce substantial and reliable physical contact between the conducting wire and the terminal. Substantial and reliable physical contact is essential for optimum electrical conductivity, and thus, the performance of the wire/terminal assembly.
There are a wide variety of methods currently employed to attach a terminal to a conducting wire. Known methods include soldering and crimping. Crimping essentially involves imposing a force on the opening of the terminal so as to deform the opening of the terminal about the conducting wire, thus securing the conducting wire therein. Crimping terminals to conducting wires is particularly desirable because the process is easily automated and results in a strong mechanical connection between the terminal and the wire.
In many cases, the crimping of the terminal to the wire has become an automated process that permits rapid production of a large number of wire/terminal assemblies. The ability to rapidly attach the terminal to the conducting wire is of particular interest in view of the large number of wire/terminal assemblies required by industries such as the automotive industry. Given the high volume of wire/terminal assemblies that are used, it has been, and remains, a desirable goal to maximize the quality and production rates by refining the crimping processes to eliminate steps and/or equipment that are inefficient or tend to slow production and/or degrade quality.
A terminal may be crimped to a conducting wire using a variety of methods. Crimping is typically accomplished by using a crimp blade and anvil configuration, collectively referred to as an `applicator die.` In operation, the opening of the terminal is placed on the anvil, and a conducting wire is inserted into the opening of the terminal. The crimp blade then descends and applies a force to the terminal opening, deforming the structure of the terminal opening around the conducting wire. The geometry of the crimp thus formed is varied by changing the shape of the contacting surfaces of the anvil and/or the crimp blade. A common geometry that is currently used is one where the crimp blade has a substantially V-shaped notch formed therein so that the wide end of the notch opens downward towards the anvil. Correspondingly, the anvil has a mating V-shaped protrusion that extends upwards and is aligned with the notch in the crimp blade.
Typically, the terminal opening comprises an open loop. The open side of the loop is placed facing upwards and the closed side of the loop reposes on the anvil. Thus, as the crimp blade descends on its downstroke, the walls of the notch contact the sides of the loop and push them inward and down around the connecting wire. The crimp blade has a surface of predefined geometry that imposes a desired shape on the crimp as the blade descends. For example, one type of a crimp blade has a surface geometry configured as to produce a crimp with a cross-section substantially in the shape of a `B.` Thus, the anvil and crimp blade cooperate to form a strong and secure mechanical connection between the terminal and the conducting wire.
While the anvil and crimp blade crimping machinery and method are generally effective, they, and other known crimping methods and machinery, suffer from a variety of significant shortcomings. One major problem with known crimping process is that the typical anvil and/or die is generally ineffectual in constraining the terminal during the crimping process itself. Accordingly, the terminal is relatively free to move and change position, with respect to the crimp blade and anvil, during the crimping process. Because close alignment between the crimp blade, anvil, and terminal is critical to a sound mechanical connection between the terminal and the connecting wire, any tendency or ability of the terminal to move during the crimping process will degrade the quality of the mechanical connection that is produced and will likely also compromise the electrical performance of the connection. For example, if multiple wires are inserted into a terminal and the terminal rotates or otherwise moves before or during crimping, some of the wires can partially withdraw from the terminal. Thus, the electrical performance of the wire/terminal assembly may be compromised. Furthermore, an out of position terminal may result in an offset crimp in which the crimped terminal grasps only a portion of the wire to which the terminal is connected. Again, such a result compromises the electrical performance and mechanical soundness of the connection, and would thus serve as grounds for rejecting the wire/terminal assembly that has been produced.
Another related problem with current methods of performing the crimping process also concerns the geometry of the known crimp blade and anvil configurations. The problem relates specifically to the relationship between the crimp blade and the anvil when the crimp blade is fully lowered to the crimping position. In particular, the crimp blade and anvil cooperate to define a space when the crimp blade is fully lowered. The shape of the space in turn defines the cross-sectional shape of the crimped wire/terminal assembly. In a typical crimp blade and anvil configuration, there is a gap between the upper corners of the anvil and the interior walls of the V-shaped notch of the crimp blade into which the anvil fits during crimping. Thus, during crimping, the crimp blade tends to push or extrude a portion of the terminal past the upper corners of the anvil and into the gap.
Extrusion of the terminal is undesirable because it results in the formation of stress cracks in the interior of the terminal. Typically such stress cracks form near the extruded portions of the terminal. Stress cracks will ultimately cause the connection to fail. This issue is of particular concern in applications such as motor vehicle air bag systems where the dependable performance of electrical components is absolutely essential. Unfortunately, the stress cracks currently cannot be detected by the visual inspection processes typically used in industry. Detecting the stress cracks requires the use of destructive inspection techniques such as cutting a cross-section through the crimp. Obviously, such destructive detection techniques would be counter-productive if applied to every wire/terminal assembly. Consequently, stress crack failures of wire/terminal assemblies often go undetected until a failed assembly is inspected.
Another significant problem with known crimping devices relates to the notch in the crimp blade. Typically, this notch describes an angle of about 3.degree.. However, because of the small angle, terminals often become stuck inside the notch after crimping. No further crimping can be accomplished with the crimp blade until the notch is cleared. Time spent clearing the notch slows down the production process and reduces the number of crimped wire/terminal assemblies that can be produced in a given time frame. Obviously, this is an undesirable result, especially in view of the need, previously noted, for a high volume production rate of these assemblies.
A related problem concerns the coating on a typical terminal. In particular, it is recognized that the mechanical and electrical performance of a wire/terminal assembly can be greatly improved by combining a soldering step with the crimping step. The terminals used in known crimping process are typically pre-coated with tin or the like so that when the crimped assembly is heated, the tin pre-coat will melt, or flow, to form a metal-to-metal bond between the terminal and the conducting wire. While pre-coating terminals is thus a desirable technique, the tin coating is problematic in the context of known crimping devices and techniques. In particular, the tin is relatively soft, as it must be in order to flow properly when heated. However, because the tin is soft and easily deformed by the crimp blade, the tinned terminals frequently stick in the crimp blade after the crimping process. Further, some of the tin coating rubs off on the blade. Consequently, after a period of time tin builds up on the blade and contributes to the sticking problem. The tin coating present on the typical terminal thus tends to exacerbate the sticking problem.
Although sticking of the wire/terminal assembly in the crimp blade is a problem that has plagued the industry for some time, current crimping machines and processing methods are generally ineffective in reducing or preventing the problem. In fact, the typical crimping machine only aggravates the situation further. Specifically, current conventional crimping devices employ a `knock-out` which is intended to forcibly eject the wire/terminal assembly from the crimp blade after crimping is completed. A gripper then withdraws the wire/terminal assembly. However, when the wire/terminal assembly is stuck in the crimp blade, the knock-out frequently fails to eject the terminal. Thus, the gripper can only withdraw the stuck wire/terminal assembly by applying an impulsive force that acts to forcibly jerk the wire/terminal assembly from the crimp blade. Unfortunately, there is a high potential that this jerking force may compromise the mechanical integrity of the connection between the wire and the terminal. Visual inspection of wire/terminal assemblies which have been forcibly jerked from the crimp blade generally fails to reveal the resulting defects. Thus, known crimping devices are particularly insidious in that they may induce defects in the wire/terminal assembly that readily escape detection. Consequently, the defect generally becomes apparent only upon analysis of a failed wire/terminal assembly.
At least one attempt has been made to prevent wire/terminal assemblies from becoming lodged in the crimp blade after crimping. In particular, known crimping processes commonly employ some type of lubrication scheme whereby a lubricant such as oil is introduced onto the surfaces of the crimp blade. While this approach is somewhat effective in eliminating the sticking problem, known lubrication methods cause some undesirable side effects. The most undesirable side effect relates to the fact that some of the lubricant used to lubricate the crimp blade inevitably finds its way into the wire/terminal assembly. The lubricant, in the context of the wire/terminal assembly, is a contaminant. Over time, the lubricant creates corrosion and a build-up of oxides between the wire and terminal which undesirably increases the resistance of the wire/terminal assembly as a whole. As with the jerking problem, the introduction of lubricant into the wire/terminal assembly ultimately causes defects that are not readily detectible at the time of assembly.
Finally, at least one other problem with known applicator dies and crimping methods is the relatively short useful life of the typical crimp blade. Typically, the crimp blade becomes worn with use, for example, by excessive scoring. Because a conventional crimp blade cannot be renewed so as to prolong its life, it must be replaced at regular intervals, usually at considerable cost.
In view of the foregoing problems with applicator dies and crimping methods that are currently available, an improved crimping process and device, particularly an improved applicator die, are needed. Specifically, the applicator die should define a crimp space geometry that precludes extrusion of the terminal and the attendant stress cracking. Further, the applicator die should securely grasp the terminal so as to prevent the terminal from rolling or moving out of position prior to or during the crimping process. Also, the applicator die should be prevent terminals from sticking in the die, and thereby obviate the need to lubricate the crimping surfaces of the crimp elements and to forcibly jerk the terminal from the applicator die after crimping. Finally, it would be desirable that the applicator die be durable and have renewable parts so as to foreclose the recurrent and expensive need to install replacement parts.